1. Field of the Invention
This disclosure generally relates to fuel cell systems and electric power plants incorporating them, and more particularly to power plants including one or more arrays of fuel cell systems.
2. Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant to electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) which includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely committed platinum, at each membrane electrode interface to induce the desired electrochemical reaction. In operation, the electrodes are electrically coupled for conducting electrons between the electrodes through an external circuit. Typically, a number of MEAs are electrically coupled in series to form a fuel cell stack having a desired power output.
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have flow passages to direct fuel and oxidant to the electrodes, namely the anode and the cathode, respectively. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant, and provide channels for the removal of reaction products, such as water formed during fuel cell operation. The fuel cell system may use the reaction products in maintaining the reaction. For example, reaction water may be used for hydrating the ion exchange membrane and/or maintaining the temperature of the fuel cell stack.
The stack's capability to produce current flow is a direct function of the amount of available reactant. Increased reactant flow increases reactant availability. The stack voltage varies inversely with respect to the stack current in a non-linear mathematical relationship. The relationship between stack voltage and stack current at a given flow of reactant is typically represented as a polarization curve for the fuel cell stack. A set or family of polarization curves can represent the stack voltage-current relationship at a variety of reactant flow rates.
In most practical applications, it is desirable to maintain an approximately constant voltage output from the fuel cell stack. One approach is to employ a battery electrically coupled in parallel with the fuel cell system to provide additional current when the demand of the load exceeds the output of the fuel cell stack and to store current when the output of the fuel cell stack exceeds the demand of the load, as taught in commonly assigned pending U.S. patent applications Ser. No. 10/017,470 entitled “Method and Apparatus for Controlling Voltage From a Fuel Cell System” Ser. No. 10/017,462 entitled “Method and Apparatus for Multiple Mode Control of Voltage From a Fuel Cell System” and Ser. No. 10/017,461 entitled “Fuel Cell System Multiple Stage Voltage Control Method and Apparatus”, all filed Dec. 14, 2001.
The many different practical applications for fuel cell based power supplies require a large variety of different power delivery capabilities. In most instances it is prohibitively costly and operationally inefficient to employ a power supply capable of providing more power than required by the application. It is also costly and inefficient to design, manufacture and maintain inventories of different power supplies capable of meeting the demand of each potential application (e.g., 1 kW, 2 kW, 5 kW, 10 kW, etc.). Further, it is desirable to increase the reliability of the power supply, without significantly increasing the cost. Thus, a less costly, less complex and/or more efficient approach to fuel cell based power supplies is desirable.